Steel superior in machinability and method of production of same

ABSTRACT

The present invention provides steel superior in machinability comprised of, by wt %, C: 0.005 to 0.2%, Si: 0.001 to 0.5%, Mn: 0.2 to 3.0%, P: 0.001 to 0.2%, S: 0.03 to 1.0%, T.N: 0.002 to 0.02%, T.O: 0.0005 to 0.035%, and the balance of Fe and unavoidable impurities, said steel satisfying one or both of Mn/S in the steel being 1.2 to 2.8 or an area ratio of pearlite over a grain size of 1 μm in a microstructure of the steel being not more than 5%.

TECHNICAL FIELD

The present invention relates to steel used for automobiles, generalmachinery, etc. and a method of production of the same, moreparticularly relates to steel superior in machinability which issuperior in tool life and cut surface roughness at the time of cuttingand chip disposal and a method of production of the same.

BACKGROUND ART

General machinery and automobiles are produced by assembling largenumbers of parts. From the viewpoint of the precision requirements andproduction efficiency, the parts are in many cases produced through acutting process. At this time, reduction of costs and improvement ofproduction efficiency are required improvement of the machinability ofthe steel is also sought. In particular, conventional SUM23 and SUM24have been developed stressing machinability. Up to now, it has beenknown that to improve the machinability, addition of S, Pb, or anothermachinability improving element is effective. However, some userssometimes avoid use of Pb due to its environmental burden. As a generaldirection, the amount of use is being reduced.

Up until now, when not adding Pb, the technique has been used ofimproving the machinability by forming inclusions such as S such as MnSbecoming soft in a cutting condition. However, a similar amount of S aswith the low carbon and sulfur free-machining steel SUM23 is added toso-called low carbon and lead free-machining steel SUM24L. Therefore, itis necessary to add an amount of S more than the past. However, withaddition of a large amount of S, if just making the MnS coarser, notonly is it necessary to obtain an MnS distribution efficient forimproving the machinability, but these form starting points of fracturein rolling, forging, etc. and cause many problems in production.Further, in sulfur free-machining steel based on SUM23, the built-upedges easily form causing relief shapes at the cut surface anddeterioration of the surface roughness accompanied with detachment ofthe built-up edges and breakoff of chips. Therefore, from the viewpointof the machinability as well, there is the problem of a drop inprecision due to the deterioration of the surface roughness. In chipdisposal as well, it is considered better that the chips be able to bebroken short, but with just simple addition of S, the ductility of thematrix is large, so sufficient breakage is not possible and no majorimprovement can be obtained.

Further, elements other than S such as Te, Bi, and P are known aselements for improving machinability, but the fact that even ifimproving the machinability to some extent, cracks easily occur at hotrolling or hot forging, so these are preferably made as low in contentas possible is disclosed in Japanese Unexamined Patent Publication(Kokai) No. 9-71840, Japanese Patent Application No. 2000-160284,Japanese Unexamined Patent Publication (Kokai) No. 2000-219936, andJapanese Unexamined Patent Publication (Kokai) No. 2001-329335.

Further, Japanese Unexamined Patent Publication (Kokai) No. 11-222646proposes a method of improving chip disposal by making the establishingthe presence of at least 30 sulfides of 20 μm or more alone or groups ofsulfides comprised of pluralities of sulfides connected substantiallylinearly in lengths of 20 μm or more in an observation field of across-section of 1 mm² in the rolling direction. However, the dispersionof sulfides of the submicron level most effective for machinability inpractice, including the method of production, is not alluded to.Further, not much can be expected in view of the ingredients as well.

Further, Japanese Unexamined Patent Publication (Kokai) No. 11-293391proposes a method of improving the chip disposal by making the averagesize of the sulfide inclusions 50 μm² or less and establishing thepresence of 750 or more sulfide inclusions per 1 mm². However, thedispersion of sulfides of the submicron level most effective formachinability in practice is not alluded to at all like in JapaneseUnexamined Patent Publication (Kokai) No. 11-222646. Further, thetechnology for deliberately creating this and the method forinvestigating this are not described either.

On the other hand, cutting tool life tends to be focused on since it hasa direct effect on the production efficiency etc., but even inmachinability, surface roughness is high in technical difficulty.Surface roughness is affected by the inherent properties of the cutmaterial, so it was difficult to obtain a surface roughness equal to orgreater than that of conventional steel. The surface roughness isdirectly linked with the performance of the part, so deterioration ofthe surface roughness becomes a cause of decline in part performance oran increase in the defect rate at the time of product production and isoften stressed more than tool life. In this sense, conventional leadfree-machining steel was superior. Compared with simple sulfurfree-machining steel, it is superior not only the tool life, but alsothe surface roughness, so much use has been made of it for preventing adrop in part performance.

In technology relating to steel for improving the surface roughness, ingeneral free-machining elements such as Pb and Bi are added. Inaddition, however, for example, as seen in Japanese Unexamined PatentPublication (Kokai) No. 5-345951, for securing a desired surfaceroughness by making the average size of the MnS inclusions finer to notmore than 50 μm², graphite free-machining steel superior in tool lifeand finished surface roughness characterized by containing graphitehaving an average cross-sectional area of 5 to 30 μm² in an amount of0.20 to 1.0% in a ferrite matrix has been seen. However, even with thesetechniques, it is difficult to obtain a surface roughness equal to orbetter than that of conventional lead free-machining steel. That is,so-called low carbon and lead free-machining steel SUM24L has beensuperior in surface roughness in the past. The reason is believed to bethat the level of fine dispersion of inclusions defined in these onlyconcerns grains of an average size of 3 μm or so, so homogeneousdispersion is insufficient and therefore built-up edges easily areformed and the surface roughness cannot be improved as much as that ofconventional lead free-machining steel.

DISCLOSURE OF INVENTION

The present invention provides steel having a good surface roughness anda method of production of the same which avoid problems in hot rollingand hot forging while improving both the tool life and surface roughnessand giving a machinability at least equivalent to that of conventionallow carbon and lead free-machining steel.

Cutting is a fracture phenomenon of breaking off chips. Promotion ofthis is one point. In particular, to obtain a good surface roughness,the inventors caused embrittlement of the matrix so as to facilitatefracture and thereby extend tool life and also suppressed nonuniformityin the steel to a minimum so as to cause a fracture phenomenon stableeven on the micro level and thereby suppress roughness of the cutsurface. Specifically, the inventors took note of the distribution ofpearlite in steel and caused C to uniformly disperse as fine pearlite(strictly speaking cementite) in steel so as to cause stable fractureand thereby create a cut surface with no roughness and provided a methodof production enabling this. The gist of the present invention is asfollows;

According to the present invention, there is provided a

-   -   (1) Steel superior in machinability comprised of, by wt %,        -   C: 0.005 to 0.2%,        -   Si: 0.001 to 0.5%,        -   Mn: 0.2 to 3.0%,        -   P: 0.001 to 0.2%,        -   S: 0.03 to 1.0%,        -   T.N: 0.002 to 0.02%,        -   T.O: 0.0005 to 0.035%, and            the balance of Fe and unavoidable impurities, said steel            satisfying one or both of Mn/S in the steel being 1.2 to 2.8            or an area ratio of pearlite over a grain size of 1 μm in a            microstructure of the steel being not more than 5% and a            surface roughness Rz of the steel being not more than 11 μm.    -   (2) Steel superior in machinability characterized by containing,        by wt %, C: 0.005% to 0.2%, Mn: 0.3 to 3.0%, and S: 0.1 to 1.0%,        by having a density of MnS having a circle equivalent diameter        of 0.1 to 0.5 μm at a cross-section parallel to a rolling        direction of the steel material, taken from an extraction        replica and observed by a transmission electron microscope, of        at least 10,000/mm², and by having a cut surface roughness Rz of        the steel of not more than 11 μm.    -   (3) Steel superior in machinability as set forth in (1) or (2),        said steel characterized by further containing B:0.0005 to 0.05        wt %.    -   (4) Steel superior in machinability as set forth in (1), said        steel characterized by having a density of MnS having a circle        equivalent diameter of 0.1 to 0.5 μm at a cross-section parallel        to a rolling direction of the steel material, taken from an        extraction replica and observed by a transmission electron        microscope, of at least 10,000/mm².    -   (5) Steel superior in machinability as set forth in (1), said        steel characterized by further restricting the amount of S to        0.25 to 0.75 wt % and the amount of B to 0.002 to 0.014 wt %, by        containing amounts of S and B in a region surrounded by A, B, C,        and D shown in FIG. 4 where the contents of S and B satisfy the        following equation (1), and by containing sulfides with BN        precipitated in MnS:        (B−0.008)²/0.006²+(S−0.5)²/0.25²≦1   (1)    -   (6) Steel superior in machinability as set forth in (1) or (2),        said steel characterized by further containing, by wt %, one or        more of,        -   V: 0.05 to 1.0%,        -   Nb: 0.005 to 0.2%,        -   Cr: 0.01 to 2.0%,        -   Mo: 0.05 to 1.0%,        -   W: 0.5 to 1.0%,        -   Ni: 0.05 to 2.0%,        -   Cu: 0.01 to 2.0%,        -   Sn: 0.005 to 2.0%,        -   Zn: 0.0005 to 0.5%,        -   Ti: 0.0005 to 0.1%,        -   Ca: 0.0002 to 0.005%,        -   Zr: 0.0005 to 0.1%,        -   Mg: 0.0003 to 0.005%,        -   Te: 0.0003 to 0.05%,        -   Bi: 0.005 to 0.5%,        -   Pb: 0.01 to 0.5%, and        -   Al: ≦0.015%.    -   (7) A method of production of steel superior in machinability as        set forth in any one of (1) to (3), said method of production of        steel characterized by casting molten steel having the steel        ingredients as set forth in (1), then cooling at a cooling rate        of 10 to 100° C./min, hot rolling, then cooling at a cooling        rate of at least 0.5° C./sec in a range from an A₃ point to 550°        C.    -   (8) A method of production of steel superior in machinability as        set forth in (4) or (5), said method of production of steel        characterized by casting molten steel having the steel        ingredients as set forth in (2), then cooling at a cooling rate        of 10 to 100° C./min, restricting a finishing temperature of hot        rolling to at least 1,000° C., then cooling at a cooling rate of        at least 0.5° C./sec in a range from an A₃ point to 550° C.    -   (9) A method of production of steel superior in machinability as        set forth in any one of (1) to (6), said method of production of        steel characterized by restricting a heating temperature for        adjusting hardness to not more than 750° C. after the cooling        after the hot rolling.    -   (10) A method of production of steel as described in any one        of (7) to (9), wherein said steel is steel superior in        machinability characterized by further containing, by wt %, one        or more of,        -   V: 0.05 to 1.0%,        -   Nb: 0.005 to 0.2%,        -   Cr: 0.01 to 2.0%,        -   Mo: 0.05 to 1.0%,        -   W: 0.5 to 1.0%,        -   Ni: 0.05 to 2.0%,        -   Cu: 0.01 to 2.0%,        -   Sn: 0.005 to 2.0%,        -   Zn: 0.0005 to 0.5%,        -   Ti: 0.0005 to 0.1%,        -   Ca: 0.0002 to 0.005%,        -   Zr: 0.0005 to 0.1%,        -   Mg: 0.0003 to 0.005%,        -   Te: 0.0003 to 0.05%,        -   Si: 0.005 to 0.5%,        -   Pb: 0.01 to 0.5%, and        -   Al: ≦0.015%.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an opticalmicrograph of a ferrite-pearlite structure of steelaccording to the present invention.

FIG. 2(a) is an opticalmicrograph of a state of fine diffusion of MnSaccording to the present invention, while FIG. 2(b) is anopticalmicrograph of a state of presence of crude MnS in conventionalsteel.

FIG. 3 is a view of the relationship of a pearlite area ratio andsurface roughness.

FIG. 4 is a view of an optimal range of an amount of S and an amount ofB according to the present invention.

FIG. 5 is a photograph of a TEM replica showing a state of sulfideshaving MnS as a main ingredient and having BN compound precipitatedaccording to the present invention.

FIG. 6 is a view of the results of EDX analysis of BN.

FIG. 7 is a view of a plunge cutting method.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is characterized by causing embrittlement of thematrix so as to obtain a sufficient machinability, in particular a goodsurface roughness, without adding lead and by adding a large amount of Bto obtain good lubrication of the contact surfaces of the tool/cutmaterial. Further, a relatively large amount of S is also added and theratio of amounts of addition of Mn and S is precisely controlled tocause them to fine disperse. Further, for the microstructure of thesteel, the pearlite seen in conventional carbon steel is controlled.That is, this is steel superior in machinability comprised of chemicalingredients, suppressed in the amount of addition of C, suppressed inthe precipitation of coarse pearlite, or, in the case of including toomuch C, suppressed in coarse pearlite grains by heat treatment, that is,suppressed in pearlite bands often seen in natural cooling.

Next, the reasons for limiting the steel ingredients defined in thepresent invention will be explained.

C is related to the basic strength of the steel and the amount of oxygenin the steel, so has a great effect on the machinability. If a largeamount of C is added to raise the strength, the machinability declines,so the upper limit was made 0.2%. On the other hand, to prevent thegeneration of hard oxides lowering the machinability and suppress thepinholes in the solidification process or other damage of dissolvedoxygen at a high temperature, it is necessary to control the amount ofoxygen to a suitable amount. If just reducing the amount of C by blowrefining, not only does the cost mount, but also a large amount ofoxygen remains in the steel and becomes a cause of pinholes and otherproblems. Therefore, the lower limit was made a 0.005% amount of C ableto easily prevent pinholes and other problems. The preferable lowerlimit of the amount of C is 0.05%.

Excessive addition of Si produces hard oxides and lowers themachinability, but suitable addition softens the oxides and does notreduce machinability. The upper limit is 0.5%. Above that, hard oxidesare produced. At 0.001% or less, softening of oxides becomes difficultand the cost increases industrially.

Mn is necessary for bonding with sulfur in the steel as MnS. Further, itis necessary to soften the oxides in the steel and make the oxidesharmless. The effect depends on the amount of S added, but if 0.2% orless, the added S cannot be sufficiently bonded as MnS and the S becomesFeS causing embrittlement. If the amount of Mn becomes large, thehardness of the base material becomes larger and the machinability andcold workability fall, so 3.0% was made the upper limit.

P causes the hardness of the base material to become greater in thesteel. Not only the cold workability, but also the hot workability andcasting properties fall, so the upper limit has to be made 0.2%. On theother hand, the lower limit value was made 0.001% by elements with theeffect of raising the machinability.

S bonds with Mn and is present as MnS inclusions. MnS improves themachinability, but stretched MnS is one cause of anisotropy at casting.Large MnS should be avoided, but addition of a large amount ispreferable from the viewpoint of improvement of the machinability.Therefore, it is preferable to cause the MnS to finely disperse. Forimprovement of the machinability to at least that of the conventionalsulfur free-machining steel in the case of no addition of Pb, additionof at least 0.03% is necessary. On the other hand, if over 1%, not onlycannot production of coarse MnS be avoided, but also cracks occur duringproduction due to deterioration of the casting properties and hotdeformation properties due to the FeS etc., so this was made the upperlimit.

B has the effect of improving the machinability when precipitated as BN.This effect is not remarkable at 0.0005% or less, while the effect issaturated even if B is added in an amount of over 0.05%. If too much BNis precipitated, conversely cracks occur during production due todeterioration of the casting properties and hot deformation properties.Therefore, the range was made over 0.0005 to 0.05%.

In the present invention, the best properties are obtained by limitingthe region surrounded by A, B, C, and D in the ellipse shown in FIG. 4strictly limited in the amount of 3 and amount of B as explained aboveto the region of equation (1):(B−0.008)²/0.006²+(S−0.5)²/0.25²≦1   (1)

N (total-N) causes the steel to harden in the case of dissolved N. Inparticular, in cutting, it hardens near the cutting edge due to dynamicstrain ageing and thereby reduces the tool life, but also has the effectof improving the cut surface roughness. Further, it bonds with B toproduce BN and improve the machinability. At 0.002% or less, no effectof improvement of the surface roughness due to dissolved nitrogen oreffect of improvement of the machinability due to BN can be observed, sothis was made the lower limit. Further, if over 0.02%, the dissolvednitrogen is present in a large amount, so conversely the tool life islowered. Further, bubbles are formed in the middle of casting and becomecauses of defects etc. Therefore, in the present invention, the upperlimit was made 0.02% where these deleterious effects become remarkable.

O (total O) forms bubbles during cooling in the case of presence in thefree state and becomes causes of pinholes. Further, control is necessaryfor softening the oxides and suppressing hard oxides harmful tomachinability. Further, oxides are utilized as nuclei for precipitationat the time of fine dispersion of MnS. If under 0.0005%, sufficient finedispersion of MnS is not possible, crude MnS is generated, and there isa detrimental effect on the mechanical properties as well, so the lowerlimit was made 0.0005%. Further, if the amount of oxygen exceeds 0.035%,bubbles form during casting to cause pinholes, so the upper limit wasmade 0.035%.

Next, the reasons for limiting the area ratio of pearlite to 5% or lesswill be explained. In general, if the steel containing carbon is cooledfrom a transformation temperature or higher, a ferrite-pearlitestructure is formed. In the case of steel containing a small amount of Ccovered by the present invention, if air cooling from a transformationtemperature (A₃ point) or more, then cutting out a piece, mirrorpolishing the inside, then etching by Nytal, it is possible to observethe microstructure as shown in FIG. 1. The black grains are compoundstructures of ferrite and cementite called “pearlite”. Normally, thegrains appearing black due to Nytal are harder than the ferrite grainsappearing white. In the deformation/fracture behavior of steel, theseexhibit behavior locally different from ferrite grains. This impairs theuniform deformation/fracture in the breakage behavior of chips atcutting, so greatly contributes to the formation of built-up edges anddegrades the surface roughness of the cut surface. Therefore, it isimportant to eliminate the structural uniformity derived from the C.Therefore, the black grains etched by Nytal are deemed to be pearlitegrains. If there are too many pearlite grains, structural uniformityoccurs and becomes a cause of deterioration of surface roughness, so thearea ratio was restricted to not more than 5% and the surface roughnessRz to not more than 11 μm. FIG. 3 shows the relationship between thepearlite area ratio and surface roughness.

Here, details of the method of measurement will be explained. The hotrolled or hot forged steel is cut to a the longitudinal cross-section(L-cross section) and buried in resin. The piece was then polished to amirror finish and etched by Nytal. The grains (circle equivalentdiameter) of 1 μm or more, except the gray MnS, in the steel etchedblack by Nytal were analyzed by an image processing system to find thearea ratio. At the time of the image processing for measurement of thearea ratio, the image contrast was adjusted by the “threshold” settingmatched with the pearlite appearing black and the inclusions appearinggray (MnS etc.) were erased from the screen so as to measure only thepearlite. The minimum pearlite detectable at this time is about 1 μm.Pearlite of less than 1 μm size does not have any effect on themachinability, so there is no effect even if not detected.

In the present invention, the measurement fields consisted of 20 fieldsof 0.2 mm² (0.4 mm×0.5 mm) at a power of ×400. The pearlite area ratiowas calculated for a total area of 4 mm².

Regarding Mn/S, it is already known that this has a large effect on thehot ductility and that normally if Mn/S>3, the production efficiency isgreatly reduced. The reason is the production of FeS. In the presentinvention, however, in the low C and high S region, the inventorsdiscovered that this ratio can be reduced to Mn/S: 1.2 to 2.8. With anMn/S of less than 1.2, a large amount of FeS is produced, the hotductility is sharply reduced, and the production efficiency is greatlyreduced.

FIG. 2 shows examples of observation of fine MnS in the cases whereMn/S≦2.8 and Mn/S>2.8 under a transmission type electron microscopeusing the replica method. When Mn/S>2.8, the result becomes only coarseMnS such as shown in FIG. 2(b) and the surface roughness cannot bereduced. On the other hand, when restricted to Mn/S:1.2 to 2.8,production of fine MnS such as shown in FIG. 2(a) is obtained.

The number of the fine MnS can be increased by repeating a process ofcontinuous casting or ingot casting, then heating to 900° C. or more.

Next, the reason for defining the density of MnS of a circle equivalentdiameter of 0.1 to 0.5 μm as at least 10,000/mm² in the type of MnS andits size and distribution will be explained.

MnS is an inclusion improving the machinability. By causing finedispersion at a high density, the machinability is remarkably improved.To obtain this effect, it is necessary that the density of MnS of acircle equivalent diameter of 0.1 to 0.5 μm be at least 10,000/mm². TheMnS sulfides are usually observed in distribution by anopticalmicroscope and measured for dimensions and density. MnS sulfidesof these dimensions cannot be confirmed by observation under anopticalmicroscope. They can only be observed first by a transmissiontype electron microscope (TEM). They are sulfides mainly comprised ofMnS of dimensions where a clear difference can be recognized under TEMobservation even if there is no difference in dimensions and densityunder observation by an opticalmicroscope. In the present invention,this is controlled and the form of presence is converted to numericalvalues to differentiate it from the prior art.

To establish the presence of MnS exceeding the above dimensions in adensity of 10,000/mm² or more, it is necessary to add a large amount ofS over the range of the present invention. If adding a large amount,there probability rises of a large number of coarse MnS also beingpresent and causing anisotropy at forging. If the MnS exceeds thisdimension due to the amount of addition of S in the range defined by thepresent invention, the amount of MnS becomes insufficient and thedensity required for improvement of the machinability can no longer bemaintained. Further, if a minimum diameter of 0.1 μm or less, there issubstantially no effect on the machinability. Therefore, it is necessarythat the density of MnS of a circle equivalent diameter of 0.1 to 0.5 μmbe at least 10,000/mm². To obtain the dimensions and density of MnS, itis more effective to not only control the cooling rate, but also makethe ratio of Mn and S contained 1.5 to 2.5.

Further, in the present invention, as shown in FIG. 5 in the above MnS,it is important that the above-mentioned MnS, as shown in FIG. 5, hasthe form of a sulfide with at least 10 wt % of boronitride (BN) compoundprecipitated.

BN normally easily precipitates at the crystal boundaries and hasdifficulty uniformly dispersing in the matrix. Therefore, it is notpossible to cause uniform embrittlement of the matrix required forimproving the machinability and not possible to sufficiently obtain theeffect of BN. For uniform dispersion in the matrix, it is necessary tocause MnS, which forms sites for precipitation of BN and is alsoeffective for improving machinability, to uniformly disperse in thematrix. By making BN and MnS compound precipitate, uniform dispersion ofBN is promoted and the machinability is greatly improved. Therefore, itis necessary that at least 10% of BN compound precipitate with MnS.

The BN referred to here, FIG. 5 showing a TEM replica photographthereof, indicates a compound of B and N where peaks of B and N areclearly recognized in EDX analysis of FIG. 6.

Note that “MnS” includes not only pure MnS, but also inclusionsincluding mainly MnS and having sulfides of Fe, Ca, Ti, Zi, Mg, REM,etc. dissolved in or bonded with the MnS for copresence, inclusions likeMnTe where elements other than S form compounds with Mn and dissolve inor bond with MnS for copresence, and the above inclusions precipitatedusing oxides as nuclei. It is a general term for Mn sulfide-typeinclusions able to be expressed by the chemical formula (Mn, X) (S, Y)(where X: sulfide forming elements other than Mn and Y: element bindingwith Mn other than S).

Next, in the present invention, in addition to the above ingredients, itis possible to add one or two or more of V, Nb, Cr, Mo, W, Ni, Sn, Zn,Ti, Ca, Zr, Mg, Te, Bi, and Pb in accordance with need.

V forms a carbonitride and can strengthen the steel by secondaryprecipitation hardening. At 0.05% or less, there is no effect on raisingthe strength, while if added in an amount over 1.0%, a large amount ofcarbonitrides is precipitated and conversely the mechanical propertiesare impaired, so this was made the upper limit.

Nb also forms a carbonitride and can strengthen the steel by secondaryprecipitation hardening. At 0.005% or less, there is no effect onraising the strength, while if added in an amount over 0.2%, a largeamount of carbonitrides is precipitated and conversely the mechanicalproperties are prevented, so this was made the upper limit.

Cr is an element improving quenchability and imparting temper softeningresistance. Therefore, this is added to steel requiring higher strength.In this case, addition of 0.01% or more is required. Further, if addedin a large amount, Cr carbides are produced, so the upper limit was made2.0%.

Mo is an element imparting temper-softening resistance and improving thequenchability. At under 0.05%, that effect cannot be detected, whileeven if added at over 1.0%, the effect is saturated, so the range ofaddition was made 0.05% to 1.0%.

W forms carbides and can strengthen the steel by secondary precipitationhardening. If 0.05% or less, there is no effect on raising the strength,while if added over 1.0%, a large amount of carbides precipitate andconversely the mechanical properties are prevented, so this was made theupper limit.

Ni strengthens the ferrite, improves the ductility, and is alsoeffective in improving the quenchability and improving the corrosionresistance. If less than 0.05%, this effect cannot be observed, whileeven if added over 2.0%, the effect is saturated in the point of themechanical properties, so this was made the upper limit.

Cu strengthens the ferrite and is effective for improving thequenchability and improves the corrosion resistance. If under 0.01%,this effect cannot be observed, while even if added over 2.0%, theeffect is saturated in the point of the mechanical properties, so thiswas made the upper limit. In particular, the hot ductility is reducedand defects are easily caused at the time of rolling, so it ispreferable to simultaneously add Ni.

Sn has the effect of causing embrittlement of ferrite, extending thetool life, and improving the surface roughness. If less than 0.005%,this effect cannot be observed, while even if added over 2.0%, theeffect is saturated in the point of the mechanical properties, so thiswas made the upper limit.

Zn has the effect of causing embrittlement of ferrite, extending thetool life, and improving the surface roughness. If less than 0.0005%,this effect cannot be observed, while even if added over 0.5%, theeffect is saturated in the point of the mechanical properties, so thiswas made the upper limit.

Ti also forms carbonitrides and strengthens the steel. Further, it is adeoxygenizing element and can form soft oxides to improve themachinability. At 0.0005% or less, that effect is not observed, whileeven if added over 0.1%, the effect becomes saturated. Further, Ti formsnitrides even at a high temperature and suppresses the growth ofaustenite grains. Therefore, the upper limit was made 0.1%. Further, Tibonds with N to form TiN, but TiN is a hard substance and reduces themachinability. Further, it reduces the amount of N is required forproducing BN effective for improving machinability. Therefore, theamount of addition of Ti is preferably made 0.010% or less.

Ca is a deoxygenizing element. It not only produces soft oxides andimproves the machinability, but also dissolves in the MnS and reducesthe transformation ability and acts to suppress elongation of the MnSshape even with rolling and hot forging. Therefore, it is an elementeffective for reducing anisotropy. If less than 0.0002%, the effect isnot remarkable, while even if adding 0.005% or more, not only does theyield become extremely poor, but also a large amount of hard CaO isproduced and conversely the machinability is reduced. Therefore, therange is defined as 0.0002 to 0.005%.

Zr is a deoxygenizing element and produces oxides. The oxides formnuclei for precipitation of MnS and are effective for the fine, uniformdiffusion of MnS. Further, it dissolves in MnS to reduce the deformationability and acts to suppress elongation of the MnS shape even with hotrolling or hot forging. Therefore, it is an element effective forreduction of anisotropy. If less than 0.0005%, the effect is notremarkable, while even if added in 0.1% or more, not only does the yieldbecome extremely poor, but also large amounts of ZrO₂, ZrS, etc. areproduced and conversely the machinability is reduced. Therefore, therange of addition was defined as 0.0005 to 0.1%. Note that when tryingto finely disperse MnS, compound addition of Zr and Ca is preferable.

Mg is a deoxygenizing element and produces oxides. The oxides formnuclei for precipitation of MnS and are effective for the fine, uniformdispersion of MnS. It is an element effective for reduction ofanisotropy. If less than 0.0003%, the effect is not remarkable, whileeven if added in 0.005% or more, not only does the yield becomeextremely poor, but also the effect is saturated. Therefore, the rangeof addition was defined as 0.0003 to 0.005%.

Te is an element for improving the machinability. Further, it producesMnTe or works with MnS to reduce the deformability of MnS and suppressthe elongation of the MnS shapes. Therefore, it is an element effectivefor reducing the anisotropy. The effect is not observed if less than0.0003%, while the effect becomes saturated if over 0.05%.

Bi and Pb are elements effective for improving machinability. Theireffects are not observed at 0.005% or less, while even if added inamounts over 0.5%, not only do the effects of improvement ofmachinability become saturated, but also the hot forgeability drops andeasily becomes a cause of defects.

Al is a deoxygenizing element and forms Al₂O₃ or AlN in steel, However,Al₂O₃ is hard, so becomes a cause of tool damage at the time of cuttingand promotes wear. Therefore, the limit was made 0.015% where a largeamount of Al₂O₃ is not produced. In particular, when giving priority totool life, the limit is preferably made 0.005% or less.

Further, in the present invention, when giving priority to avoidingtrouble in quenching rather than machinability, it is possible to reducethe amount of B in the allowable range of machinability. For example, bymaking the amount of B in the composition of ingredients defined by thepresent invention 0.0005 to 0.005% and making the amount of S 0.5 to 1.0wt %, it is possible to obtain steel superior in machinability. This isbecause if B is present in a large amount, the dissolved B remains, sothe hardened layer becomes too deep due to the carburization quenchingor other heat treatment, so by increasing the strain in the partperformance or making the hardened parts brittle, it is possible toprevent various types of trouble such as quench cracks. Further, in thepresent invention, in cold forging, wire drawing, and other methods ofworking other than machining seen in free-machining steel, MnS easilybecomes starting points of fractures. The mechanical propertiessometimes are reduced due to the occurrence of cracks. Therefore, tosecure the minimum extent of machinability of the free-machining steel,it is possible to suppress the amount of S to 0.03 to 0.5 wt % so as tosuppress cold forging and high frequency surface layer cracks.

Next, the method of production of steel for causing fine dispersion ofMnS and BN in the above way will be explained.

The fine dispersion of sulfides having MnS as a main ingredient andhaving BN compound precipitated is effective for improvement of themachinability. To get the sulfides finely dispersed, it is necessary tocontrol the precipitation of the sulfides having MnS as a mainingredient and having BN compound precipitated. For this control, it isnecessary to define the range of cooling rate during casting. With acooling rate of 10° C./min or less, the solidification is too slow andthe sulfides having MnS as a main ingredient and having BN compoundprecipitated end up becoming coarser and can no longer be finelydispersed. With a cooling rate of 100° C./min or more, the density ofthe fine sulfides produced becomes saturated, the hardness of the billetrises, and the danger of cracks increases. The cooling rate can beeasily obtained by controlling the size of the cross-section of thecasting mold, the casting speed, etc. to suitable values. This may beapplied to the continuous casting method and the pouring method.

The “cooling rate” referred to here means the speed at the time ofcooling from the liquid phase line temperature to the solid phase linetemperature in the billet thickness direction Q part. The cooling rateis found by calculation by the following equation from the secondarydendrite arm spacing of the solidified structure in the billet thicknessdirection after solidification.${Rc} = \left( \frac{\lambda\quad 2}{770} \right)^{\frac{1}{0,41}}$

where, Rc: cooling rate (° C./min)

-   -   λ2: secondary dendrite arm spacing (μm)

That is, since the secondary dendrite arm spacing changes depending onthe cooling conditions, it is possible to measure this to confirm thecontrolled cooling rate.

BN dissolves in austenite at 1000° C. or more. At a temperature of 1000°C. or less, the BN precipitated in the process from the casting to therough rolling remains at the grain boundaries and compound precipitationas sulfides having MnS as a main ingredient and having BN compoundprecipitated is not possible. By rolling at a temperature of 1000° C. ormore in the finishing (final) rolling step at the hot rolling, the oncedissolved BN easily compound precipitates as nuclei for precipitation ofMnS sulfides. If finally rolling at 1000° C. or less, compoundprecipitation of sulfides mainly comprised of BN and MnS no longereasily occurs.

Next, the method of production for obtaining a microstructure of apearlite area ratio of 5% or less in the present invention will beexplained.

The behavior of formation of built-up edges on tools has a great effecton the cut surface roughness. Inherently, dynamatically speaking, thearea right above the cutting tool is the harshest environment formaterials and fracture/breakage of materials easily occur, so thereshould be no formation of built-up edges. In practice, built-up edgesare formed due to the powerful adhesion between the tool and cutmaterial and the structural uniformity of the cut material. Therefore,it is considered important to greatly increase the homogeneity of themicrostructure of the material. As a result, the inventors discoveredthat the pearlite distribution, which had been considered almostirrelevant up to now, is greatly related to the homogeneity of themicrostructure.

Here, the “pearlite” means a structure appearing black when etching amirror polished surface by Nytal. “Pearlite” strictly speaking indicatesferrite and plate-shaped cementite alternately arranged. Under anopticalmicroscope, a single crystal grain appears to be seen. Further,as shown in FIG. 1, with production by normal rolling and cooling, thepearlite grains precipitate in band shapes (hereinafter referred to as“pearlite bands”). This pearlite differs in mechanical properties fromthe single phase ferrite of the matrix, so the deformation and fracturenear the cutting edge become uneven and further the growth of built-upedges is augmented.

Therefore, the inventors adjusted the steel ingredients or thermalhistory to suppress the area ratio of pearlite grains of a grain size of1 μm or more in an observation field of a measurement field of 4 mm² andinvestigate the critical region where a good surface roughness isobtained, whereupon they learned that deterioration of the surfaceroughness is suppressed by making the area ratio of pearlite grains of 1μm or more a ratio of not more than 5%. FIG. 2 shows the relationshipbetween the area ratio of pearlite and the surface roughness.

As shown in FIG. 1, it is learned that the free-machining steelaccording to the present invention has extremely little of such astructure appearing black. In the present invention, the result isstrictly speaking tempered martensite or tempered bainite. Thepossibility cannot be denied that the carbides are not pearlite (inother words, a striped structure of plate-shaped cementite and ferrite),but cementite grains. However, here, such ferrous carbides will bereferred to all together as “pearlite”.

Next, the method of production of free-machining steel according to thepresent invention will be explained.

[Thermal history quenching: 0.5° C./s from temperature of A₃ point ormore to 550° C. or less]

In the present invention, as the thermal history after hot rolling, itis important to cool from a temperature of above the A₃ point after hotrolling to 550° C. or less by a cooling rate of at least 0.5° C./sec.

In the past, the practice had been to rapidly cool so-called low carbonfree-machining steel. Low carbon free-machining steel is low in amountof C, so even with quenching, there is little change in hardness.Therefore, there is no effect on the strength/toughness due toconventional “quenching and tempering” and the fixed idea that this isnot necessary for free-machining steel is not bound to. However, whenpursuing homogeneity of the quality considering the nature of cutting,it is sufficient to rapidly cool from the A₃ point so as to freezemovement of C in the steel and suppress the generation of coarsecementite and pearlite occurring due to the transformation at the timeof air cooling. In this case, since the hardening due to quenching isnot the objective, even if not becoming a quenched structure having amartensite structure, it is sufficient to freeze movement of C in thesteel and prevent the generation of coarse cementite or pearlite.Therefore, as shown in FIG. 3, it is necessary to cool from the A₃ pointto 550° C. or less by a rate of 0.5° C./sec or more. When the content ofthe elements improving quenchability is low, a cooling rate of at least1° C./s is preferable. If the temperature after cooling exceeds 550° C.or the cooling rate is slower than 0.5, coarse pearlite is produced. Ingeneral, this is precipitated in band shapes referred to as pearlitebands. Naturally, if the alloy elements are added in large amounts aswith stainless steel, even if the cooling rate is slower than 0.5°C./sec, pearlite bands are not formed. Here, however, generalfree-machining steel is envisioned, so the cooling rate is defined as0.5° C./sec.

Next, in the present invention, after the above rapid cooling, heattreatment for holding at a temperature of 750° C. or less may beperformed to make the structure of the free-machining steel morehomogeneous.

In the actual production process, to further increase the stability ofthe product, while the amount of C is small, it is preferable to reducethe variation in hardness in the steel. Therefore, it is possible toagain hold the steel at a high temperature so as to reduce the variationin the material. First, to suppress the coarse pearlite, it is importantto rapidly cool from a temperature of the A₃ point or more to 550° C. orless where coarse pearlite is no longer produced. On top of this, asshown in FIG. 4, it is possible to retain the steel again at apredetermined temperature T₂° C. to adjust the hardness to onesatisfying user requirements and reduce the variation in hardness aswell. By heating and retention at a temperature of not more than 750°C., the steel is adjusted to a hardness satisfying the requirements of auser.

Regarding the holding temperature T₂° C., the holding temperature andthe holding time should be determined so as to give a hardnesssatisfying the demands of the users. However, if the holding temperatureT₂° C. exceeds 750° C., transformation to austenite starts, so if thecooling rate at cooling again is slow, pearlite bands end up beingproduced. Therefore, the holding temperature T₂° C. was made 750° C. orless. Further, wire drawing or other secondary working is often appliedat a later step, so it is preferable to adjust the temperature T₂° C. soas to give a hardness suitable for handling in the later step. Regardingthe holding time, industrially speaking, at 3 minutes or less, there isalmost no change in hardness etc. compared with almost no holding, sothe time is preferably made at least this.

Note that in industrial production, the temperature becomes uneven evenin the steel due to the rolling or forging dimensions etc., so theholding time at the temperature T1° C. of up to 550° C. after rapidcooling for preventing coarse pearlite should also be considered. Byholding at a temperature T1° C. of 550° C. or less after rapid coolingfor preferably at least 5 minutes, uniform ferrite transformation can bepromoted without relation to the dimensions of the material orsegregation bands. By doing this, after this, even if raising thetemperature to the holding temperature T₂° C. (≦750° C.), coarsepearlite or pearlite bands will not be generated. Conversely, when thedimensions after rolling or forging are large, if the holding time at550° C. or less is shorter than 1 minute, the internal transformationdoes not end, so coarse pearlite or pearlite bands are produced ifholding at a temperature of 550° C. or more after that.

EXAMPLES Example 1

The effect of the present invention will be explained by examples. Amongthe test materials shown in Table 1, Table 2 (continuation 1 of Table1), Table 3 (continuation 2 of Table 1), Table 4 (continuation 3 ofTable 1), Table 5 (continuation 4 of Table 1), and Table 6 (continuation5 of Table 1), No. 13 was melted in a 270 t converter, while the restwere melted in a 2 t vacuum melting furnace, then the materials werebloomed into billets and rolled to φ60 mm.

In the section on heat treatment in the tables, the examples marked as“Normal.” are held at 920° C. for at least 10 min and then air-cooled.The examples of the invention marked as “QT” are inserted into a watertank at the rear end of the rolling line and rapidly cooled from 920°C., then held by annealing at 700° C. for at least 1 hour. The pearlitearea ratio was adjusted by this. In the invention examples, steels witha low amount of C can be reduced in area ratio of pearlite even withnormalization.

The machinability of the material shown in Examples 1 to 81 of Table 1to Table 6 was evaluated by a drilling test of the conditions shown inTable 7. The machinability was evaluated at the maximum cutting speed(so-called VL1000, unit m/min) enabling cutting up to a cumulative holedepth of 1000 mm.

The cut surface roughness showing the surface quality in the cutting wasevaluated. The cutting conditions are shown in Table 8 and the method ofevaluation (hereinafter referred to as a “plunge cutting test”) is shownin FIG. 7(a) and FIG. 7(b). In the plunge cutting test, a tool isrepeatedly used for cutting for a short period. With one cuttingoperation, the tool does not move in the longitudinal direction of themachined material, but moves toward the center of the rotating cutmaterial, so the tool is pulled back after a short time of cutting. Theshape is basically the shape of the built-up edge of the tooltransferred to the surface of the cut material. The surface roughness ofthe cut surface transferred is affected by formation of the built-upedges or the wear loss of the tool. The surface roughness was measuredby a surface roughness meter. The 10-point surface roughness Rz (μm) wasused as an indicator of the surface roughness.

Invention Examples 1 to 7 were all superior in drill life compared withComparative Examples 76 to 81 and were good in surface roughness inplunge cutting. This is believed to be because the B caused the ferriteto be locally made brittle and the surface was smoothly created, so agood surface roughness was obtained.

The effect of improvement of surface roughness was remarkable when S wasover 0.5% but an effect was seen in chip disposal even when the amountof S was smaller.

Further, an effect was recognized even when the ratio of Mn and S wasthe 3 or so often seen in conventional steel, but if Mn/S is madesmaller, the tool life is improved more and the surface roughness isalso improved. The reason is that in an environment with a large amountof B added, the fine MnS finely disperses even in the ferrite andeffectively functions for both the lubrication effect and theembrittlement effect. However, if Mn/S is too small such as with Example80, FeS is produced, so roll cracks occur. In the evaluation of thepresent invention, Example 70 had roll cracks, so could not be evaluatedfor machinability etc. at all, so the results of evaluation were notrecorded in the tables.

Even if changing the amount of C somewhat (Tables 1 to 6 and Examples 37to 75), a good tool life and cut surface roughness could be obtained byadding a large amount of B and by controlling the area ratio ofpearlite.

Note that regarding the chip disposal, it is preferable that the chipsbe small in curvature at the time of curling or that they be broken.Therefore, chips extending long curled 3 or more turns by a radius ofcurvature over 20 mm are deemed defective. Chips with a large number ofturns and small radius of curvature or chips with a large radius ofcurvature and length not reaching 100 mm are deemed good. TABLE 1Chemical ingredients wt % Ex. Class C Si Mn P S B total-N total-O V NbCz Mo W Ni Cu Sn Zn 1 Inv. ex. 0.023 0.004 1.69 0.072 0.52 0.0080 0.00790.0187 2 Inv. ex. 0.011 0.015 2.05 0.077 0.72 0.0067 0.0061 0.0174 3Inv. ex. 0.055 0.008 1.64 0.078 0.55 0.0094 0.0096 0.0202 4 Inv. ex.0.058 0.013 2.36 0.077 0.75 0.0098 0.0102 0.0152 5 Inv. ex. 0.101 0.0091.62 0.080 0.52 0.0062 0.0055 0.0153 6 Inv. ex. 0.090 0.009 2.14 0.0880.75 0.0110 0.0127 0.0206 7 Inv. ex. 0.118 0.005 1.71 0.076 0.53 0.00500.0040 0.0164 8 Inv. ex. 0.117 0.007 2.10 0.079 0.73 0.0109 0.01160.0175 9 Inv. ex. 0.167 0.004 1.70 0.083 0.55 0.0089 0.0090 0.0200 10Inv. ex. 0.174 0.007 2.19 0.072 0.75 0.0216 0.0126 0.0200 11 Inv. ex.0.065 0.009 1.66 0.089 0.52 0.0129 0.0142 0.0166 12 Inv. ex. 0.055 0.0041.70 0.074 0.58 0.0130 0.0143 0.0171 13 Inv. ex. 0.057 0.012 1.75 0.0780.57 0.0133 0.0147 0.0169 14 Inv. ex. 0.058 0.013 1.84 0.084 0.59 0.01270.0139 0.0056 15 Inv. ex. 0.057 0.005 1.76 0.078 0.56 0.0052 0.00420.0157 0.11 16 Inv. ex. 0.053 0.005 1.70 0.078 0.60 0.0094 0.0096 0.01550.032 17 Inv. ex. 0.055 0.013 1.72 0.083 0.55 0.0131 0.0146 0.0207 0.3418 Inv. ex. 0.050 0.014 1.91 0.078 0.58 0.0105 0.0111 0.0174 0.21 19Inv. ex. 0.055 0.010 1.68 0.089 0.56 0.0051 0.0042 0.0196 0.11 0.48 20Inv. ex. 0.057 0.013 1.49 0.082 0.51 0.0128 0.0131 0.0182 0.21 21 Inv.ex. 0.057 0.010 1.77 0.072 0.58 0.0053 0.0043 0.0164 0.36 22 Inv. ex.0.050 0.005 1.75 0.087 0.54 0.0069 0.0064 0.0153 0.0040 23 Inv. ex.0.054 0.012 1.57 0.080 0.53 0.0099 0.0104 0.0079 24 Inv. ex. 0.050 0.0091.81 0.089 0.55 0.0075 0.0073 0.0052 25 Inv. ex. 0.050 0.014 1.80 0.0790.58 0.0081 0.0079 0.0185 26 Inv. ex. 0.057 0.014 1.72 0.081 0.53 0.00830.0083 0.0208 27 Inv. ex. 0.051 0.014 1.76 0.078 0.60 0.0112 0.01190.0170 28 Inv. ex. 0.055 0.003 1.67 0.090 0.55 0.0101 0.0107 0.0191 29Inv. ex. 0.050 0.003 1.83 0.089 0.56 0.078 0.0077 0.0210 30 Inv. ex.0.057 0.013 1.70 0.073 0.59 0.0080 0.0079 0.0059 31 Inv. ex. 0.022 0.0100.98 0.079 0.54 0.0097 0.0101 0.0190 32 Inv. ex. 0.020 0.007 1.63 0.0740.76 0.0123 0.0134 0.0168 33 Inv. ex. 0.059 0.007 1.43 0.089 0.59 0.01250.0137 0.0184 34 Inv. ex. 0.052 0.003 1.64 0.085 0.73 0.0063 0.00570.0207 35 Inv. ex. 0.099 0.015 1.24 0.074 0.52 0.0131 0.0145 0.0190 d

TABLE 2 (continuation 1 of Table 1) Pearlite Surface Chemicalingredients (wt %) Heat area VL1000 roughness Chip Ex. Class Ti Ca Zr MgTe Bi Pb Al Mn/S treatment ratio (%) m/min Rz (μm) disposal 1 Inv. ex.0.0011 3.26 Normal. 1.5 147 10.5 G 2 Inv. ex. 0.0013 2.84 Normal. 0.6155 10.4 G 3 Inv. ex. 0.0023 2.98 QT 1.9 144 7.3 G 4 Inv. ex. 0.00183.13 QT 0.7 157 6.6 G 5 Inv. ex. 0.0013 3.13 QT 0.7 142 7.8 G 6 Inv. ex.0.0021 2.83 QT 2.0 152 6.2 G 7 Inv. ex. 0.0019 3.24 QT 2.0 147 6.6 G 8Inv. ex. 0.0020 2.88 QT 1.4 157 7.4 G 9 Inv. ex. 0.0017 3.11 QT 2.6 1416.8 G 10 Inv. ex. 0.0013 2.91 QT 0.6 145 6.5 G 11 Inv. ex. 0.0020 3.19Normal. 5.5 130 10.8 G 12 Inv. ex. 0.0017 2.92 QT 2.3 131 6.4 G 13 Inv.ex. 0.0026 3.08 QT 2.7 126 6.3 G 14 Inv. ex. 0.0024 3.14 QT 0.8 145 7.5G 15 Inv. ex. 0.0025 3.13 QT 2.6 146 7.7 G 16 Inv. ex. 0.0023 2.84 QT0.7 144 6.6 G 17 Inv. ex. 0.0012 3.14 QT 2.8 147 6.8 G 18 Inv. ex.0.0025 3.29 QT 0.5 145 7.5 G 19 Inv. ex. 0.0025 3.01 QT 1.5 147 7.0 G 20Inv. ex. 0.0023 2.89 QT 2.5 145 7.0 G 21 Inv. ex. 0.0016 3.03 QT 3.0 1466.9 G 22 Inv. ex. 0.0011 3.24 QT 0.8 143 7.2 G 23 Inv. ex. 0.026 0.00302.96 QT 1.0 143 8.0 G 24 Inv. ex. 0.0037 0.0028 3.26 QT 1.3 145 7.2 G 25Inv. ex. 0.0037 0.0021 3.09 QT 3.0 144 6.9 G 26 Inv. ex. 0.0025 0.00273.25 QT 2.9 146 7.7 G 27 Inv. ex. 0.0030 0.0022 2.94 QT 1.0 144 7.9 G 28Inv. ex. 0.16 0.0012 3.02 QT 1.2 170 7.3 G 29 Inv. ex. 0.283 0.0018 3.29QT 1.3 170 6.4 G 30 Inv. ex. 0.0153 2.88 QT 0.8 128 7.1 G 31 Inv. ex.0.0029 1.82 Normal. 1.4 154 10.2 G 32 Inv. ex. 0.0030 2.16 Normal. 1.4165 11.7 G 33 Inv. ex. 0.0013 2.42 QT 2.0 156 3.9 G 34 Inv. ex. 0.00202.25 QT 1.4 167 4.5 G 35 Inv. ex. 0.0027 2.39 QT 0.7 153 4.1 G

TABLE 3 (continuation 2 of Table 1) Chemical ingredients wt % Ex. ClassC Si Mn P S B total-N total-O V Nb Cr Mo W Ni Cu Sn Zn 36 Inv. ex. 0.0910.006 1.54 0.079 0.77 0.0057 0.0050 0.0168 37 Inv. ex. 0.115 0.013 1.340.072 0.56 0.0202 0.0107 0.0194 38 Inv. ex. 0.118 0.011 1.61 0.083 0.760.0090 0.0091 0.0297 39 Inv. ex. 0.167 0.007 1.36 0.089 0.57 0.00520.0042 0.0166 40 Inv. ex. 0.171 0.006 1.42 0.089 0.71 0.0097 0.01000.0191 41 Inv. ex. 0.064 0.007 1.15 0.086 0.59 0.0121 0.0132 0.0208 42Inv. ex. 0.053 0.003 1.00 0.074 0.53 0.0104 0.0110 0.0172 43 Inv. ex.0.052 0.014 1.13 0.077 0.58 0.0095 0.0098 0.0160 44 Inv. ex. 0.056 0.0141.04 0.089 0.54 0.0082 0.0081 0.0109 45 Inv. ex. 0.053 0.013 1.06 0.0770.59 0.0065 0.0059 0.0172 0.10 46 Inv. ex. 0.050 0.007 1.14 0.088 0.570.0115 0.0124 0.0181 0.038 47 Inv. ex. 0.053 0.009 1.26 0.082 0.530.0094 0.0097 0.0185 0.67 48 Inv. ex. 0.058 0.006 1.13 0.076 0.54 0.00560.0047 0.0173 0.22 49 Inv. ex. 0.059 0.002 1.20 0.090 0.60 0.0090 0.00910.0192 0.48 50 Inv. ex. 0.057 0.005 1.31 0.082 0.56 0.0055 0.0046 0.01710.12 51 Inv. ex. 0.051 0.002 1.15 0.070 0.57 0.0076 0.0072 0.0186 0.240.0027 52 Inv. ex. 0.050 0.012 v25 0.079 0.55 0.0085 0.0085 0.0157 53Inv. ex. 0.055 0.014 1.26 0.074 0.60 0.0109 0.0116 0.0058 54 Inv. ex.0.055 0.003 0.99 0.073 0.52 0.0070 0.0066 0.0103 55 Inv. ex. 0.059 0.0111.09 0.087 0.51 0.0129 0.0142 0.0175 56 Inv. ex. 0.052 0.003 1.07 0.0820.59 0.0063 0.0057 0.0187 57 Inv. ex. 0.056 0.010 1.17 0.075 0.53 0.0630.0057 0.0165 58 Inv. ex. 0.051 0.004 1.27 0.072 0.53 0.0126 0.01380.0189 59 Inv. ex. 0.056 0.010 1.12 0.080 0.56 0.0123 0.0134 0.0173 60Inv. ex. 0.052 0.011 1.03 0.087 0.53 0.0113 0.0121 0.0087 61 Inv. ex.0.056 0.008 1.46 0.079 0.54 0.0087 0.0100 0.0049 62 Inv. ex. 0.051 0.0091.65 0.077 0.56 0.0089 0.0099 0.0045 63 Inv. ex. 0.056 0.006 1.45 0.0820.54 0.0098 0.0099 0.0020 64 Inv. ex. 0.061 0.007 1.40 0.081 0.57 0.00890.0091 0.0123 65 Inv. ex. 0.071 0.011 1.10 0.002 0.55 0.0087 0.00950.0110 66 Inv. ex. 0.060 0.010 1.20 0.078 0.60 0.0103 0.0124 0.0112 67Inv. ex. 0.060 0.009 1.06 0.077 0.53 0.0110 0.0121 0.0100 68 Inv. ex.0.060 0.009 1.08 0.076 0.54 0.0092 0.0112 0.0101 69 Inv. ex. 0.070 0.0081.40 0.086 0.56 0.0088 0.0095 0.0157 70 Inv. ex. 0.061 0.010 1.53 0.0770.61 0.0104 0.0124 0.0058 d 71 Inv. ex. 0.060 0.060 1.35 0.077 0.540.0110 0.0122 0.0189

TABLE 4 (continuation 3 of Table 1) Pearlite Surface Chemicalingredients (wt %) Heat area VL1000 roughness Chip Ex. Class Ti Ca Zr MgTe Bi Pb Al Mn/S treatment ratio (%) m/min Rz (μm) disposal 36 Inv. ex.0.0028 2.01 QT 3.0 168 3.5 G 37 Inv. ex. 0.0018 2.39 QT 2.2 154 3.4 G 38Inv. ex. 0.0014 2.11 QT 2.1 170 3.7 G 39 Inv. ex. 0.0024 2.39 QT 0.5 1563.5 G 40 Inv. ex. 0.0027 2.00 QT 0.7 168 3.9 G 41 Inv. ex. 0.0014 1.95Normal. 5.2 135 3.9 G 42 Inv. ex. 0.0023 1.90 QT 2.5 131 3.6 G 43 Inv.ex. 0.0029 1.95 QT 2.0 133 3.1 G 44 Inv. ex. 0.0016 1.92 QT 1.0 155 3.4G 45 Inv. ex. 0.0015 1.82 QT 2.8 156 3.7 G 46 Inv. ex. 0.0026 2.00 QT1.9 155 3.3 G 47 Inv. ex. 0.0012 2.39 QT 1.4 156 3.7 G 48 Inv. ex.0.0026 2.09 QT 0.6 155 3.6 G 49 Inv. ex. 0.0012 2.00 QT 2.8 154 4.1 G 50Inv. ex. 0.0030 2.31 QT 1.4 156 4.2 G 51 Inv. ex. 0.0019 2.02 QT 2.6 1553.3 G 52 Inv. ex. 0.0029 2.27 QT 0.8 153 4.8 G 53 Inv. ex. 0.036 0.00162.12 QT 1.3 156 4.7 G 54 Inv. ex. 0.0033 0.0017 1.89 QT 2.5 156 4.5 G 55Inv. ex. 0.0035 0.0024 2.14 QT 2.1 154 3.0 G 56 Inv. ex. 0.0020 0.00131.82 QT 2.6 154 4.3 G 57 Inv. ex. 0.0061 0.0022 2.21 QT 2.4 154 3.6 G 58Inv. ex. 0.16 0.0017 2.37 QT 2.8 182 2.6 G 59 Inv. ex. 0.266 0.0031 2.02QT 2.5 189 2.2 G 60 Inv. ex. 0.0280 1.96 QT 1.9 136 3.5 G 61 Inv. ex.0.0010 2.70 QT 2.3 146 6.5 G 62 Inv. ex. 0.005 0.0009 0.0021 2.95 QT 3.4145 6.4 G 63 Inv. ex. 0.0022 0.0025 0.0010 2.68 QT 2.9 245 6.6 G 64 Inv.ex. 0.0018 0.0012 0.0011 2.45 QT 3.0 139 6.5 G 65 Inv. ex. 0.0016 2.00QT 2.5 172 7.$$ G 66 Inv. ex. 0.0030 0.0015 2.00 QT 2.8 134 6.5 G 67Inv. ex. 0.0012 2.00 QT 3.6 131 8.9 G 68 Inv. ex. 0.0025 0.0015 0.00192.00 QT 2.1 130 6.1 G 69 Inv. ex. 0.0016 2.50 QT 3.9 135 9.9 G 70 Inv.ex. 0.0017 2.51 QT 2.3 133 7.2 G 71 Inv. ex. 0.0025 0.0010 2.50 QT 3.9132 6.5 G

TABLE 5 (continuation 4 of Table 1) Chemical ingredients wt % Ex. ClassC Si Mn P S B total-N tolal-O V Nb Cz Mo W Ni Cu Sn Zn 72 Inv. ex. 0.0590.009 1.38 0.075 0.55 0.0092 0.0132 0.0173 73 Inv. ex. 0.069 0.009 1.620.076 0.54 0.0089 0.0095 0.0160 74 Inv. ex. 0.062 0.006 1.80 0.090 0.600.0100 0.0106 0.0181 75 Comp. ex. 0.058 0.002 1.65 0.079 0.55 0.01100.0122 0.0173 76 Comp. ex. 0.045 0.007 1.00 0.084 0.35 0.0076 0.00740.0183 77 Comp. ex. 0.050 0.005 1.79 0.074 0.59 0.0067 0.0062 0.0180 78Comp. ex. 0.049 0.008 0.96 0.077 0.34 0.0129 0.0141 0.0205 79 Comp. ex.0.055 0.009 1.78 0.080 0.59 — 0.0123 0.0151 80 Comp. ex. 0.047 0.0110.48 0.085 0.53 0.0089 0.0090 0.0167 81 Comp. ex. 0.048 0.008 0.93 0.0890.53 — 0.0139 0.0151

TABLE 6 (continuation 1 of Table 1) Pearlite Surface Chemicalingredients (wt %) Heat area VL1000 roughness Chip Ex. Class Ti Ca Zr MgTe Bi Pb Al Mn/S treatment ratio (%) m/min Rz {μm} disposal 72 Inv. ex.0.0016 2.51 QT 2.2 132 7.2 G 73 Inv. ex. 0.0016 0.0010 0.0006 3.00 QT2.6 134 9.1 G 74 Inv. ex. 0.0010 3.00 QT 1.9 130 8.2 G 75 Inv. ex.0.0022 0.0017 0.0009 2.00 QT 2.9 130 6.4 G 76 Comp. ex. 0.0012 2.90Normal. 5.8  97 17.0 P 77 Comp. ex. 0.0013 3.05 Normal. 5.8 119 21.1 G78 Comp. ex. 0.0017 2.83 Normal. 5.8 100 24.4 G 79 Comp. ex. 0.0011 3.03Normal. 5.3 119 24.2 G 80 Comp. ex. 0.0013 0.90 — — — — — 81 Comp. ex.0.0027 2.81 Normal. 5.9 117 24.5 P

TABLE 7 Cutting conditions Drill Others Cutting speed: 80 m/min φ5 mmNACHI Hole depth: 15 mm Feed: 0.05 mm/rev ordinary drill, Tool life:Until Insoluble projection amount breakage machining oil 60 mm

TABLE 8 Cutting conditions Tool Others Cutting speed: 80 m/min SKH57equivalent Projection Feed: 0.05 mm/rev Rake angle: 20° EvaluationInsoluble Relief angle: 6° timing: 200 machining oil cycles

Example 2

Parts of the test materials shown in Table 9, Table 10 (continuation 1of Table 9), Table 11 (continuation 2 of Table 9), Table 12(continuation 3 of Table 9), Table 13 (continuation 4 of Table 9), andTable 14 (continuation 5 of Table 9) were produced by a 270 t converter,then casted at a cooling rate of 10 to 100° C./min. The billet wasbloomed, then further rolled to φ50 mm. Further, the rest was melted ina 2 t vacuum melting furnace and rolled to φ50 mm. At this time, thecooling rate of the billet was adjusted by changing the cross-sectionaldimensions of the casting mold. The machinability of the material wasevaluated by a drilling test of the conditions shown in Table 7 andplunge cutting of the conditions shown in Table 8. The drill boring testis a method evaluating the machinability by the maximum cutting speed(so-called VL1000, unit m/min) enabling cutting up to a cumulative holedepth of 1000 mm. Plunge cutting is a method of evaluating the surfaceroughness by transferring a tool shape by a cutting tool. Theexperimental method is shown in FIG. 7(a) and FIG. 7(b). In thisexperiment, the surface roughness in the case of cutting 200 grooves wasmeasured by a surface roughness meter. The 10-point surface roughness Rz(unit: μm) was used as an indicator of the surface roughness.

The density of the sulfides mainly comprised of Mns of dimensions of acircle equivalent diameter of 0.1 to 0.5 μm density was measured bytaking a sample by the extraction replica method from the Q part of thecross-section parallel to the rolling direction after rolling to φ50 mmand observing it under a transmission type electronmicroscope. Themeasurement was conducted by observing at least 40 fields of 80 μm² at×10000 power and converting to the number of sulfides mainly comprisedof MnS per square mm. The steels with the calculated values of equation(1) of Table 10, Table 12, and Table 14 are development steelssatisfying the present invention.

As shown in FIG. 2(a) and FIG. 2(b), MnS of a size which cannot beconfirmed at the opticalmicroscope level is clearly different indimensions and density in the inventions of the examples and comparativeexamples by observation of TEM replicas.

Note that the cutting resistance and chip disposal of Table 10, Table12, and Table 14 are as follows. The cutting resistance was measured byattaching a piezoelectric dynamometer (made by Kistler) to the turret ofa lathe, setting the tool on it to give the same position as normalcutting, and performing plunge cutting. Due to this, measurement ispossible using the main force component and back force component appliedto the tool as voltage signals. The cutting speed, feed speed, and othercutting conditions are similar to those for evaluation of the cutsurface roughness.

Regarding chip disposal, it is preferable that the chips be small incurvature at the time of curling or that they be broken. Therefore,chips extending long curled 3 or more turns by a radius of curvatureover 20 mm are deemed defective. Chips with a large number of turns andsmall radius of curvature or chips with a large radius of curvature andlength not reaching 100 mm are deemed good.

In machinability, the examples of the present invention were superior indrill tool life compared with any of the comparative examples and weregood in surface roughness at plunge cutting. In particular, it waspossible to obtain an extremely superior value of surface roughness bythe effect of compound precipitation of the fine MnS and BN. TABLE 9Chemical ingredients (wt %) Cl St'1 C Si Mn P S Total N Total O B V NbInv. 1 0.051 0.012 0.83 0.076 0.56 0.0140 0.0202 0.0070 ex. 2 0.0330.003 0.76 0.084 0.52 0.0124 0.0153 0.0066 3 0.021 0.005 1.05 0.079 0.540.0044 0.0177 0.0061 4 0.052 0.010 0.91 0.075 0.47 0.0148 0.0157 0.00595 0.053 0.009 1.45 0.071 0.61 0.0125 0.0184 0.0079 6 0.021 0.012 1.310.077 0.62 0.0051 0.0207 0.0079 7 0.053 0.005 1.72 0.077 0.60 0.00440.0202 0.0077 8 0.021 0.014 1.31 0.081 0.46 0.0113 0.0187 0.0068 9 0.0570.013 1.07 0.080 0.54 0.0126 0.0181 0.0070 0.10 10 0.055 0.008 1.100.078 0.56 0.0051 0.0175 0.0079 0.005 11 0.052 0.011 1.17 0.079 0.590.0082 0.0202 0.0056 12 0.051 0.006 1.15 0.080 0.58 0.0121 0.0209 0.006613 0.029 0.010 0.93 0.089 0.48 0.0118 0.0194 0.0053 14 0.059 0.012 0.900.077 0.46 0.0110 0.0190 0.0057 15 0.055 0.005 0.98 0.076 0.50 0.00690.0208 0.0066 16 0.021 0.008 1.03 0.087 0.52 0.0078 0.0200 0.0078 170.031 0.010 0.90 0.088 0.48 0.0067 0.0158 0.0054 18 0.052 0.004 0.890.078 0.45 0.0071 0.0181 0.0073 19 0.053 0.011 0.95 0.086 0.49 0.01200.0190 0.0073 20 0.023 0.008 1.04 0.077 0.53 0.0135 0.0205 0.0079 210.039 0.002 1.09 0.061 0.55 0.0128 0.0151 0.0062 22 0.051 0.008 1.050.076 0.54 0.0102 0.0208 0.0051 23 0.053 0.008 1.11 0.083 0.57 0.00770.0162 0.0078 24 0.029 0.010 0.98 0.088 0.50 0.0065 0.0184 0.0057 250.053 0.004 1.13 0.080 0.57 0.0169 0.0109 0.0066 26 0.051 0.011 1.040.077 0.53 0.0092 0.0160 0.0076 27 0.065 0.005 0.67 0.087 0.46 0.01520.0165 0.0050 28 0.064 0.010 0.75 0.082 0.52 0.0048 0.0161 0.0075 290.111 0.010 1.03 0.071 0.53 0.0053 0.0200 0.0056 30 0.055 0.014 1.120.080 0.57 0.0064 0.0162 0.0075 Chemical ingredients (wt %) Cl St'1 CrMo W Ni Cu Su Zn Ti Ca Inv. 1 ex. 2 3 4 5 6 7 8 9 10 11 0.41 12 0.36 130.10 0.23 14 0.11 0.28 15 0.28 16 0.23 17 0.03 0.0065 18 0.0100 19 0.03820 0.0018 21 22 23 24 25 26 27 28 29 30

TABLE 10 (continuation 1 of Table 9) Cooling Rolling Chemical speed atfinishing ingredients (wt %) casting temp. Class St'1 Zr Mg Te Bi Pb Al[° C./min] (° C.) Inv. 1 0.002 100 1097 ex. 2 0.004 72 1073 3 0.004 641020 4 0.003 55 1035 5 0.003 47 1029 6 0.002 34 1055 7 0.002 37 1079 80.001 92 1031 9 0.004 66 1176 10 0.004 14 1104 11 0.005 37 1098 12 0.00228 1181 13 0.002 82 1173 14 0.005 88 1096 15 0.003 97 1145 16 0.003 671101 17 0.001 39 1165 18 0.003 77 1116 19 0.002 87 1012 20 0.0020 0.00286 1001 21 0.0038 0.003 92 1153 22 0.0029 0.0026 0.002 54 1103 23 0.00200.006 82 1124 24 0.256 0.005 38 1129 25 0.16 0.002 80 1018 26 0.001 951199 27 0.002 77 1131 28 0.003 20 1173 29 0.002 47 1089 30 0.004 91 1133TEM BN Cutting replica comp. resistance (N) Cal. MnS prec. Surface BackMain val. density rate VL1000 roughness force force Chip of eq. ClassSt'1 (/mm²) (%) (m/min) {μmRz} comp. comp. disp. (1) Inv. 1 353565 20145 6.7 65 390 G 0.09 ex. 2 249998 15 149 5.4 73 342 G 0.06 3 328542 29142 7.0 86 358 G 0.13 4 262595 25 148 4.1 64 383 G 0.14 5 166778 16 1498.9 87 385 G 0.19 6 178854 29 133 8.4 72 352 G 0.23 7 148887 12 142 7.471 332 G 0.16 8 305248 28 140 7.9 67 339 G 0.07 9 299171 18 131 5.2 84331 G 0.05 10 82353 22 136 5.9 90 350 G 0.06 11 186895 16 141 8.8 80 368G 0.29 12 142954 28 140 4.6 83 342 G 0.16 13 384851 27 144 4.5 72 381 G0.21 14 394447 20 132 4.4 62 336 G 0.17 15 432218 18 141 5.0 67 367 G0.05 16 260532 26 139 4.4 72 380 G 0.01 17 120677 22 143 6.7 62 342 G0.19 18 266882 12 137 4.2 78 355 G 0.05 19 407007 21 135 5.8 69 377 G0.02 20 333280 11 148 6.1 73 346 G 0.01 21 366185 12 147 4.5 69 380 G0.13 22 303000 23 138 5.3 69 367 G 0.26 23 285444 24 147 4.3 62 379 G0.08 24 243854 10 134 6.1 74 360 G 0.15 25 365823 22 145 5.6 66 332 G0.13 26 309532 10 139 4.7 75 387 G 0.02 27 255448 13 134 6.7 83 363 G0.28 28 146979 20 145 4.3 84 366 G 0.01 29 260872 18 145 8.9 66 332 G0.17 30 281096 22 145 6.9 65 369 G 0.09

TABLE 11 (continuation 2 of Table 9 Chemical ingredients (wt %) Cl. St'1C Si Mn P S Total N Total O B V Nb Cr Mo W Ni Cu Su Zn Ti Ca Inv. 310.116 0.003 1.37 0.073 0.55 0.0119 0.0208 0.0078 ex. 32 0.077 0.004 1.390.070 0.56 0.0089 0.0168 0.0060 33 0.071 0.007 1.32 0.084 0.46 0.01350.0154 0.0063 34 0.102 0.013 1.36 0.088 0.48 0.0140 0.0177 0.0077 350.054 0.003 1.69 0.073 0.56 0.0133 0.0163 0.0067 36 0.056 0.007 1.570.075 0.55 0.0139 0.0183 0.0060 37 0.159 0.011 0.74 0.084 0.51 0.01150.0194 0.0054 38 0.176 0.004 0.73 0.072 0.50 0.0147 0.0167 0.0059 390.177 0.014 0.97 0.071 0.49 0.0053 0.0177 0.0075 40 0.182 0.004 1.040.080 0.53 0.0105 0.0166 0.0053 41 0.150 0.004 1.29 0.073 0.49 0.01240.0189 0.0056 42 0.199 0.012 1.42 0.087 0.57 0.0120 0.0174 0.0075 430.189 0.015 1.30 0.073 0.45 0.0104 0.0160 0.0076 44 0.165 0.010 1.330.080 0.46 0.0148 0.0209 0.0067 45 0.171 0.007 1.34 0.077 0.47 0.01770.0156 0.0078 46 0.191 0.009 1.56 0.089 0.55 0.0112 0.0153 0.0065 470.051 0.008 1.03 0.086 0.51 0.0110 0.0050 0.0072 0.005 48 0.031 0.0031.03 0.078 0.52 0.0100 0.0185 0.0115 0.0020 49 0.053 0.004 1.02 0.0800.53 0.0103 0.0159 0.0078 0.0019 50 0.084 0.008 1.01 0.082 0.52 0.00840.0040 0.0112 51 0.065 0.006 1.01 0.081 0.46 0.0110 0.0152 0.0100 520.057 0.008 1.03 0.080 0.53 0.0109 0.0156 0.0132 53 0.049 0.008 1.050.082 0.50 0.0112 0.0125 0.0112 54 0.079 0.010 0.99 0.072 0.47 0.01130.0145 0.0108 55 0.082 0.008 1.34 0.080 0.67 0.0106 0.0121 0.0035 560.064 0.010 1.12 0.079 0.50 0.0112 0.0134 0.0105 0.006 57 0.055 0.0101.15 0.074 0.49 0.0108 0.0127 0.0114 0.0015 58 0.070 0.010 1.20 0.0710.51 0.0112 0.0184 0.0112 0.0018 59 0.076 0.009 0.81 0.077 0.30 0.01110.0147 0.0121 60 0.081 0.008 1.34 0.079 0.64 0.0109 0.0156 0.0121

TABLE 12 (continuation 3 of Table 9) TEM BN Cutting Cooling Rollingreplica comp. Surface resistance (N) Cal. speed at finishing MnS prec.rough- Back Main val. Chemical ingredients (wt %) casting temp. densityrate VL1000 ness force force Chip of eq. Cl. St'1 Zr Mg Te Bi Pb Al (°C./min) (° C.) (/mm²) (%) (m/min) (μmRz) comp. comp. disp. (1) Inv. 310.003 16 1057 85221 14 132 7.6 82 386 G 0.04 ex. 32 0.002 45 1120 14273815 147 7.9 79 338 G 0.18 33 0.002 16 1017 61245 10 149 7.0 65 371 G 0.1134 0.003 78 1110 272514 28 133 7.8 70 349 G 0.01 35 0.17 0.002 77 1168262609 15 135 4.9 63 344 G 0.10 36 0.298 0.002 21 1106 81541 18 146 5.061 335 G 0.15 37 0.003 52 1100 194907 16 145 5.5 73 351 G 0.19 38 0.00259 1085 301851 15 132 6.9 80 378 G 0.13 39 0.001 22 1191 125206 30 1456.7 74 382 G 0.01 40 0.003 74 1125 262061 11 135 5.0 75 358 G 0.21 410.003 23 1036 108319 19 144 7.6 67 331 G 0.16 42 0.002 50 1163 170214 17133 8.7 87 379 G 0.09 43 0.003 11 1171 50750 25 137 6.7 67 366 G 0.04 440.004 69 1098 234200 10 138 7.0 83 388 G 0.07 45 0.286 0.004 53 1095289829 14 148 6.8 89 332 G 0.02 46 0.20 0.003 53 1089 186791 22 147 6.080 333 G 0.10 47 0.002 89 1011 416010 26 140 5.5 66 354 G 0.02 48 0.00180.001 85 1000 333350 13 144 6.2 72 344 G 0.35 49 0.0021 0.001 86 1003353921 12 139 6.1 70 352 G 0.02 50 0.0010 0.003 20 1173 146542 22 1454.5 84 366 G 0.29 51 0.002 78 1130 253458 21 145 4.0 81 352 G 0.23 520.001 79 1126 262337 20 140 4.1 82 362 G 0.77 53 0.001 65 1002 189562 20140 4.1 82 345 G 0.28 54 0.001 82 1121 252563 21 135 4.4 84 361 G 0.2355 0.001 54 1056 164512 20 140 4.1 81 361 G 1.02 56 0.001 77 1096 13265417 135 5.1 82 375 G 0.17 57 0.0012 0.001 78 1059 192563 14 135 5.6 84375 G 0.32 58 0.0014 0.001 62 1100 189562 15 135 5.7 81 352 G 0.29 590.0011 0.001 50 1058 123654 16 140 4.9 86 362 G 1.11 60 0.001 51 1123165842 14 135 5.2 83 374 G 0.78

TABLE 13 (continuation 4 of Table 9) Chemical ingredients (wt %) Cl.St'l C Si Mn P S Total N Total O B V Nb Cr Mo W Ni Cu Su Zn Ti Ca Inv.61 0.060 0.008 1.45 0.080 0.65 0.0112 0.0132 0.0050 ex. 62 0.061 0.0110.75 0.076 0.33 0.0104 0.0112 0.0110 63 0.068 0.008 1.51 0.081 0.580.0132 0.0156 0.0110 64 0.072 0.009 0.71 0.072 0.30 0.0122 0.0125 0.011065 0.082 0.008 0.88 0.077 0.34 0.0118 0.0135 0.0043 Comp. 66 0.081 0.0030.93 0.077 0.31 0.0099 0.0170 ex. 67 0.072 0.010 0.75 0.076 0.24 0.00690.0184 68 0.097 0.017 0.90 0.072 0.30 0.0095 0.0175 69 0.067 0.006 0.920.077 0.30 0.0142 0.0168 70 0.069 0.011 0.843 0.088 0.28 0.0130 0.017771 0.089 0.012 0.37 0.070 0.12 0.0103 0.0191 72 0.092 0.019 0.31 0.0790.11 0.0166 0.0174 73 0.096 0.014 0.40 0.089 0.13 0.0173 0.0177 74 0.0640.035 0.94 0.070 0.01 0.0133 0.0158 0.0035 75 0.079 0.036 0.50 0.0710.17 0.0126 0.0178 0.0013 76 0.090 0.012 0.34 0.081 0.12 0.0167 0.01830.0030 77 0.089 0.015 0.98 0.073 0.32 0.0134 0.0205 0.0038

TABLE 14 (continuation 5 of Table 9) TEM Cutting Cooling replica BNresistance (N) Cal. speed Rolling Mns comp. Surface Back Main val. ofChemical ingredients (wt %) at casting finishing density prec. VL1000roughness force force Chip eq. Class St'l Zr Mg Te Bi Pb Al (° C./min)temp. (° C.) (/mm²) rate (%) (m/min) (μmRz) comp. comp. disp. (1) Inv.61 0.002 71 1005 212365 16 140 5.0 81 366 G 0.61 ex. 62 0.001 70 1022196354 14 140 6.2 86 379 G 0.71 63 0.002 56 1006 156235 20 145 5.1 82354 G 0.35 64 0.001 69 1215 142562 19 140 4.9 83 362 G 0.89 65 0.001 721231 212365 17 135 5.1 85 374 G 0.79 Comp. 66 0.004 6 865 232 0 92 17.7173 451 P 2.36 ex. 67 0.004 7 820 194 0 95 19.4 169 512 P 2.82 68 0.0025 784 214 0 66 18.2 188 452 G 2.45 69 0.001 2 831 53 0 83 15.5 201 466 G2.41 70 0.002 5 814 192 0 99 15.4 217 497 P 2.54 71 0.001 8 763 227 0 7318.7 210 454 P 4.03 72 0.003 4 799 161 0 79 18.5 155 524 G 4.24 73 0.0043 821 141 0 66 19.9 189 464 G 3.95 74 0.002 8 844 207 0 75 17.8 152 500P 4.39 75 0.001 2 774 57 0 93 16.9 209 481 P 3.02 76 0.003 6 891 180 193 17.9 217 486 G 3.07 77 0.004 6 827 154 1 83 15.3 199 523 G 1.10

INDUSTRIAL APPLICABILITY

As explained above, the present invention enables use for automobileparts and general machinery parts have superior properties of tool lifeand cut surface roughness at the time of cutting and disposal of chips.

1. Steel superior in machinability comprised of, by wt %, C: 0.005 to0.2%, Si: 0.001 to 0.5%, Mn:.0.2 to 3.0%, P: 0.001 to 0.2%, S: 0.03 to1.0%, T.N: 0.002 to 0.02%, T.O: 0.0005 to 0.035%, and the balance of Feand unavoidable impurities, said steel satisfying one or both of Mn/S inthe steel being 1.2 to 2.8 or an area ratio of pearlite over a grainsize of 1 μm in a microstructure of the steel being not more than 5% anda surface roughness Rz of the steel being not more than 11 μm.
 2. Steelsuperior in machinability characterized by containing, by wt %, C:0.005% to 0.2%, Mn: 0.3 to 3.0%, and S: 0.1 to 1.0%, by having a densityof MnS having a circle equivalent diameter of 0.1 to 0.5 μm at across-section parallel to a rolling direction of the steel material,taken from an extraction replica and observed by a transmission electronmicroscope, of at least 10,000/mm², and by having a cut surfaceroughness Rz of the steel of not more than 11 μm.
 3. Steel superior inmachinability as set forth in claim 1 or 2, said steel characterized byfurther containing B:0.0005 to 0.05 wt %.
 4. Steel superior inmachinability as set forth in claim 1, said steel characterized byhaving a density of MnS having a circle equivalent diameter of 0.1 to0.5 μm at a cross-section parallel to a rolling direction of the steelmaterial, taken from an extraction replica and observed by atransmission electron microscope, of at least 10,000/mm².
 5. Steelsuperior in machinability as set forth in claim 1, said steelcharacterized by further restricting the amount of S to 0.25 to 0.75 wt% and the amount of B to 0.002 to 0.014 wt %, by containing amounts of Sand B in a region surrounded by A, B, C, and D shown in FIG. 4 where thecontents of S and B satisfy the following equation (1), and bycontaining sulfides with BN precipitated in MnS:(B−0.008)²/0.006²+(S−0.5)²/0.25²≦1   (1)
 6. Steel superior inmachinability as set forth in claim 1 or 2, said steel characterized byfurther containing, by wt %, one or more of, V: 0.05 to 1.0%, Nb: 0.005to 0.2%, Cr: 0.01 to 2.0%, Mo: 0.05 to 1.0%, W: 0.5 to 1.0%, Ni: 0.05 to2.0%, Cu: 0.01 to 2.0%, Sn: 0.005 to 2.0%, Zn: 0.0005 to 0.5%, Ti:0.0005 to 0.1%, Ca: 0.0002 to 0.005%, Zr: 0.0005 to 0.1%, Mg: 0.0003 to0.005%, Te: 0.0003 to 0.05%, Bi: 0.005 to 0.5%, Pb: 0.01 to 0.5%, andAl: ≦0.015%.
 7. A method of production of steel superior inmachinability, said method of production of steel characterized bycasting molten steel having the steel ingredients as set forth in claim1, then cooling at a cooling rate of 10 to 100° C./min, then cooling ata cooling rate of at least 0.5° C./sec in a range from an A₃ point to550° C.
 8. A method of production of steel superior in machinability,said method of production of steel characterized by casting molten steelhaving the steel ingredients as set forth in claim 2, then cooling at acooling rate of 10 to 100° C./min, restricting a finishing temperatureof hot rolling to at least 1,000° C., then cooling at a cooling rate ofat least 0.5° C./sec in a range from an A₃ point to 550° C.
 9. A methodof production of steel superior in machinability as set forth in claim 7or 8, said method of production of steel characterized by restricting aheating temperature for adjusting hardness to not more than 750° C.after the cooling after the hot rolling.
 10. A method of production ofsteel as described in claim 7 or 8, wherein said steel is steel superiorin machinability characterized by further containing, by wt %, one ormore of, V: 0.05 to 1.0%, Nb: 0.005 to 0.2%, Cr: 0.01 to 2.0%, Mo: 0.05to 1.0%, W: 0.5 to 1.0%, Ni: 0.05 to 2.0%, Cu: 0.01 to 2.0%, Sn: 0.005to 2.0%, Zn: 0.0005 to 0.5%, Ti: 0.0005 to 0.1%, Ca: 0.0002 to 0.005%,Zr: 0.0005 to 0.1%, Mg: 0.0003 to 0.005%, Te: 0.0003 to 0.05%, Bi: 0.005to 0.5%, Pb: 0.01 to 0.5%, and Al: ≦0.015%.